Custom memcached service name

[CS-Tao]

The same issue is #169. But I don't think the solution #169 (comment) is reasonable.

Custom memcached service name by environment :

version: '2.0'
services:
  seafile-db:
    image: mariadb:10.1
  ...

  seafile-memcached:
    image: memcached:1.5.6
  ...

  seafile:
    image: seafileltd/seafileenvi-mc:latest
    environment:
      - DB_HOST=seafile-db
      - MEM_HOST=seafile-memcached
  ...

[svengreb]

I've posted a workaround in #86 (comment) . It makes use of a Docker network alias which allows to use a custom service name for the memcached container, but ensures the hardcoded memcached host name must not be changed within the source code itself.
Note that this is just a workaround and that a configuration or environment variable is still the more correct way to solve this.